The present invention pertains to a pressure-measurement device of the type specified in the preamble of claim 1.
A partial cross section of a known pressure-measurement device is shown in FIG. 1. Such pressure-measurement devices have at their end face, between the diaphragm seal housing 20 of diaphragm seal 2 and the base body 10 of pressure sensor 1, an elastomer gasket 3. A fluid, silicone oil or hydraulic oil is typically provided as the pressure transfer medium between diaphragm seal 2 and measuring diaphragm 11 of pressure sensor 1, and is filled into the pressure transducer in a well-degassed and nearly water-free state.
The problematic feature of such sensors, however, is the increasing leakiness of the elastomer gasket between diaphragm seal and pressure sensor. Over time and especially as temperatures rise, an increased gas permeability of the elastomer gasket occurs with such sensors. This leakiness of the elastomer gasket then leads to an ever-increasing measurement error. The gas permeability of the elastomer gasket results in an exchange of gas with the environment, which may cause so much gas to penetrate from the environment into the pressure sensor and to become dissolved in the pressure transfer medium that the vapor pressure of the gases dissolved in this pressure transfer medium corresponds to the external pressure of the gases against the elastomer gasket. If the gas in the pressure transfer medium escapes again, for instance, during depressurization, and thus fills the interior of the diaphragm seal, then an undefined elevated and unstable pressure signal results which is measured by the pressure sensor and makes accurate and reliable measurement of pressure impossible.
In addition to gas, of course, water vapor from, for instance, humid air can also penetrate through the elastomer gasket into the pressure transducer. At temperatures above 100xc2x0 C., the vapor pressure of the infiltrated water is greater than atmospheric pressure and likewise leads to the previously mentioned elevation of internal pressure in the sensor and thus to measurement errors.
The present invention is therefore based on the problem of providing a pressure-measurement device of the initially mentioned type which has a better seal between diaphragm seal and pressure sensor, especially at higher temperatures.
According to the invention, this problem is solved by a pressure transducer with the characteristics of claim 1.
According to the latter, a generic pressure-measurement device is provided which is characterized in that the pressure sensor has a connection part made of a ceramic material that is coupled on the diaphragm side to the measuring diaphragm via a second diffusion-tight joint and is coupled on the diaphragm-seal side via a third diffusion-tight joint to the diaphragm seal.
It is guaranteed by the present invention that the measuring diaphragm is sealed on both the pressure sensor side and the diaphragm seal side by way of respective diffusion-tight joints. It is assured in this way that no gases or water vapor can reach the pressure-transfer medium. Thus the operational mode of the pressure transducer remains functional over a long time, and particularly at high temperatures.
The joints between measuring diaphragm and connection part or base body are preferably embodied as glass solder joints. These glass solder rings exhibit the advantage that they are electrical insulators, are temperature-resistant even at high temperatures, and do not permit any diffusion of gases or hydrogen from the outside to the inside.
In an advantageous configuration, an adapter is provided between the connection part and the diaphragm seal housing, and is joined to the latter by flanges and diffusion-tight joints.
It is particularly advantageous if the adapter and the connection part have identical or very similar coefficients of thermal expansion. In a typical configuration, the base body as well as the connection part and the adapter consist of a ceramic material, i.e., they each have a similar coefficient of thermal expansion of roughly 8xc3x9710xe2x88x926/K.
In a preferred embodiment of the invention, the base body and/or the connection part and/or the adapter and/or the measuring diaphragm consist of an oxide material such as Al2O3 ceramic, SiC ceramic, glass ceramic, quartz or ZrO2 ceramic.
The adapter is expediently joined, stress-free, with hard solder via an annular flange to the connection part of the pressure sensor. Since the connection part and the adapter typically feature [sic; consist of] a material with similar or identical coefficients of expansion, a joint between these parts that is stable over the long term and diffusion-tight is thus guaranteed. On its end face opposite the diaphragm seal housing, the adapter is welded by a weldment joint to a projecting flange, consisting of special steel of the diaphragm seal housing. The differing coefficients of expansion between the adapter and the diaphragm seal housing are equalized here by the weld. The annular peripheral weld typically has a smaller diameter than the annular flange for the hard solder joint. Thereby, strains between the pressure sensor and the diaphragm seal can be reduced by the weld joint.
An oil, such as hydraulic oil or silicon oil, is typically employed as the pressure-transfer medium.
The pressure sensor is advantageously constructed as a capacitive pressure sensor or as a DMS pressure sensor. In this case, either the film electrode forms the measuring diaphragm or a circular or annular film electrode is formed on the measuring diaphragm. The other film electrode of the measuring capacitor is then arranged in the chamber between measuring diaphragm and base body, with a common, arc-resistant gas being used as the dielectric.
Additional advantageous configurations and refinements of the invention can be derived from the subordinate claims, the description below and the figures.